Chimney Height Calculator (3-2-10 Rule)
Two clearances, one taller-wins answer: how high the chimney top has to sit above the roof and anything near it.
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The 3-2-10 rule is a max() of two clearances — at least 3 ft above where the chimney passes through the roof, AND at least 2 ft higher than any roofline, ridge or wall within 10 ft. Here the top must reach 5.0 ft above the datum, set by the 2-ft clearance over a nearby ridge/wall. A short chimney or a nearby ridge starves the draft.
The 3-2-10 rule is the one every inspector recites, and it is just a max() of two clearances. The chimney top has to finish at least 3 feet above the point where it pokes through the roof, AND at least 2 feet above the highest thing — ridge, gable, parapet, wall — standing within a 10-foot horizontal reach. Whichever of those two is higher is the height you build to.
It exists for draft, not looks. A stub chimney tucked below a nearby ridge sits in a pocket of positive pressure when the wind blows over the roof, and that stalls or reverses the flow. Enter both heights off a common reference point and the tool tells you the required top — and which clearance is doing the work.
Formula
Pick a datum (the roof surface at the penetration is easiest), then:
- Clearance A
= roof penetration height + 3 ft - Clearance B
= tallest point within 10 ft + 2 ft - Required top
= max(A, B)
The “10” is a horizontal radius, measured flat, not along the slope. Only things inside that circle count toward clearance B.
Worked example
Say the chimney exits low on a steep roof and the ridge is 8 feet away horizontally, rising 5.5 ft above the exit. Set the penetration as the datum (0). Clearance A = 0 + 3 = 3 ft. Clearance B = 5.5 + 2 = 7.5 ft. The top must reach 7.5 ft above the datum — the nearby ridge governs, not the 3-foot minimum. Build to the 3-foot rule alone and you would be 4.5 feet short and fighting the draft every windy night.
Measure the radius flat, take the taller clearance
Measure horizontally, then vertically. The 10 feet is a flat radius on the plan, and the 2-foot clearance is checked against whatever is tallest inside that circle. Watch for:
- Measuring the 10 ft up the slope instead of across — a steep roof exaggerates it.
- Ignoring a taller adjacent structure — a neighbor’s wall or a second gable counts if it is within reach.
- Confusing minimum height with good height. Taller usually drafts better, within reason and local code.
Reference table
The same rule, four roofs. The taller of the two clearances wins — a nearby ridge or wall is what usually forces the extra height (heights are feet above a common datum).
| Roof situation | Roof penetration | Tallest within 10 ft | Required top |
|---|---|---|---|
| Low-slope roof, nothing tall within 10 ft | 0.0 ft | 0.0 ft | 3.0 ft |
| Chimney a few feet down-slope from the ridge | 0.0 ft | 3.0 ft | 5.0 ft |
| Steep roof, ridge close and high within 10 ft | 0.0 ft | 5.5 ft | 7.5 ft |
| Tall neighboring gable wall within 10 ft | 1.0 ft | 6.0 ft | 8.0 ft |
Frequently asked questions
What is the 3-2-10 rule for chimneys?
A chimney must terminate at least 3 ft above the roof where it passes through, and at least 2 ft above any part of the roof, ridge or wall within 10 ft of it, measured horizontally. You build to the taller of those two.
Is the 10 feet measured along the roof or straight across?
Straight across — it is a horizontal radius on the plan view. On a steep roof the along-the-slope distance is longer, so measuring the wrong way makes you overbuild.
Does a nearby tree or the neighbor’s house count?
The rule is written around the roof and structures on it. A wall or gable within 10 ft counts. Trees and separate buildings are a draft-and-wind judgment for a pro, not part of the code minimum.
My chimney meets 3 feet but still draws poorly. Why?
Check the 2-foot clearance against anything within 10 ft — a ridge just outside arm’s reach is the classic culprit. Height, flue size and a cold exterior chimney all interact; see the draft calculator.